Rabbit liver microsomes provide 4 electrophoretically distinctive forms of cytochrome P-450, LM1, LM2, LM4 and LM7 molecular weight of which are 47,000, 48,700, 55,300 and 60,000 daltons respectively. LM2 is a predominant form induced by phenobarbital, LM4 predominant form induced by -naphthoflavone even all forms are detectable in uninduced rabbit microsomes as constituents of multiple forms of cytochrome P-450. The cytochrome P-450 is the enzyme responsible for mixed function oxidase (aryl hydrocarbon hydroxylase) which activates benzo(a)pyrene to ultimate carcinogen, benzo(a)pyrene-7,8-diol-9,10-epoxide with the function of epoxide hydratase. Monoclonal antibody to P-450 LM2 made by hybridoma between myeloma and mouse spleen cells derived from mice immunized with P-450 LM2 specifically binds, precipitates and inhibits the enzyme activity of P-450 LM2 but not P-450 1M1, LM4 and LM7. The purpose of this work is to investigate the ability of monoclonal antibodies to distinguish specifically the different forms of cytochrome P-450.